1. Field of the Invention
This invention relates generally to the field of electroplating. The invention relates more specifically to a method of electroplating, and a layer of electroplated material deposited therewith, that are suitable for deposition on a low conductivity substrate material.
2. Description of the Related Art
Various difficulties are associated with the electroplating of metals such as copper onto low conductivity barrier materials such as Ta and W. In the context of semiconductor fabrication, the major problem associated with plating on such barriers is that of achieving the required adhesion and uniformity of the electroplated layer across the surface of a wafer without the presence of fill defects.
Pulse plating, such as that disclosed in U.S. Pat. No. 5,972,192 to Dubin et al., has historically been employed to plate difficult-to-plate materials or shapes. While conventional pulse plating techniques can enable conformal plating across a surface under certain circumstances, for the following reason these techniques are not always effective for plating a resistive layer.
The electrical current is proportional to the voltage applied at a particular point (generally described by the Butler-Volmer Equation) as follows:i=nFAko[Co(0,t)e−αnf(E-Eo′)−Cr(0,t) e−(1−α)nf(E-Eo′)].
Since the current is logarithmic with the applied potential (typically called overpotential E-Eo′, where E is the applied voltage and Eo′ is the formal potential defining the thermodynamic equilibrium point in a particular electrolyte), the potential needed to be applied to the edge of an object in order to plate metal at the center of the object is well above that which is needed for plating at the edge of the object near the contact point. The waveform creates zones of excess plating toward the outside of the object, optimal growth rate in a finite region of the object, and no plating at the center of the object during the typical waveform (excluding the initial amplitudes). Therefore, with conventional techniques, the electroplated deposits are typically excessively thick at the edge of an object, such as a wafer, with minimal deposition at the center. Typical electroplating for wafers is accomplished with a dielectric material placed between the anode and cathode to modify the electric field.
In an attempt to overcome the non-uniform deposition, a high pulse amplitude technique has been employed. While the use of a high pulse amplitude may provide a more uniform deposit, it will also lead to filling problems in the high aspect ratio features common to semiconductor processes. To overcome such filling problems, the use of a current reversal waveform can be employed. The current reversal waveform can deplate metal from the regions that are thicker, or deplate the thicker regions more quickly than the thin center portions, and therefore increase the fill of high aspect ratio features. For example, U.S. Pat. No. 6,071,398 to Martin et al. discloses a method of pulse plating in which the ratio of peak reverse current density to peak forward current density is varied in periodic cycles. Martin, which focuses on achieving bottom up fill, discloses that the ratio is varied sequentially between first, second, and third values.
Electroplating a layer of metal on a layer of low conductivity material, however, presents another obstacle. With the low conductivity material, the IR drop across the surface of the low conductivity material means that the filling is limited to a small portion of the surface where the potential is defined by a narrow window.
Therefore, a need exists for a method of electroplating which not only provides for the uniform filling of high aspect ratio features, but which also provides for the controlled deposition of a layer of desired structure and thickness across the entire surface of a low conductivity material.